Bone sialoprotein
(BSP)

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Gene: maps to 4q28-q31 (Kerr J.M. et al., 1993). It spans approximately 15 kb and comprises seven exons (six small -51 to 159 bp- and 1 large -approximately 2.6 kb-) and six introns. The first exon encodes the 5'-UTR, the second exon the signal sequence and the first two amino acids, exons 3 and 4 the Tyr-and Phe-rich amino terminus, and exon 5 the first segment of polyglutamic acid. Exon 7 encodes over half of the protein including a second polyglutamic acid segment, the RGD cell attachment motif, the sulphated tyrosine-rich C-terminus and the 3'-UTR. The promoter region is characterized by an inverted TATA-like sequence (TTTATA) and an inverted CCAAT box (ATTGG). Analysis of chimeric constructs fused to a CAT reporter gene indicate that the presence of both the inverted TATA-like sequence and CCAAT elements are required for basal promoter activity (Kim R.H. et al., 1994).
mRNA: size: kb
Protein: an acidic, phosphorylated (ser/thr) and sulfated (tyr) glycoprotein (approximately 70 kDa) known to be secreted by bone cells. 317 amino acids, including a 16-amino acid hydrophobic signal sequence (Fisher L.W. et al., 1990). BSP is thought to take part in bone formation by binding to hydroxyapatite, the bone mineral, through two to three stretches of polyglutamic acid. However, a role of BSP in bone resorption has also been suggested from in vitro results (Raynal C. et al., 1996).

Cell lines:
- Proliferative and adhesive responses of MDA-MB-231 breast cancer cells (BCC) to BSP or BSP fragments have been attributed to interactions between the alpha_Vbeta₅ integrin expressed by BCC and the RGD (Arg-Gly-Asp) tripeptide (and adjacent regions) contained in BSP. In contrast, migrational response of BCC to BSP appeared to involve alpha_Vbeta₃ and to be only partly associated with RGD interactions (Sung V. et al., 1998).

- Despite its abundance in carcinomas, BSP expression was not detected in a panel of 11 human BCC lines (MCF-7, T47D, SK-Br-3, MDA-MB-453, MDA-MB-231, MDA-MB-436, BT549, MCF-7ADR, Hs578T, MDA-MB-435, and LCC15-MB) and osteopontin (OPN) expression was detected only in two of these (MDA-MB-435 and LCC15-MB). To examine the possibility that expression of these genes was down-regulated in cell culture, several cell lines were grown as nude mouse xenografts in vivo; however, these tumors also failed to express BSP. OPN expression was identified in all cell lines grown as nude mouse xenografts (Sharp J.M. et al., 1999).

Tumors:
- BSP has been localized to human breast cancer in association with abnormal microcalcifications. Its expression in primary breast tumors has been correlated with bone metastasis development and poor survival (Bellahcene A. et al., 1995, 1996a, 1996b).

- 18 primary breast cancers were studied by RT-PCR, for expression of calcitonin receptor (CTR) and of the bone proteins osteopontin (OPN) and bone sialoprotein (BSP). OPN and CTR were expressed by each of the tumours, and 7 (39%) additionally expressed an alternate form of CTR, whilst BSP was expressed by 13 tumours (72%). In situ hybridisation confirmed that expression of OPN and CTR was confined to the tumour cells (Gillespie M.T. et al., 1997).

- BSP transcripts were detected in 65% and osteopontin (OPN) transcripts in 77% of a cohort of archival, primary invasive breast carcinoma examined. In general, BSP and OPN transcripts were detected in both invasive and in situ carcinoma components. The transcripts were not detected in surrounding stromal cells or in peritumoral macrophages (Sharp J.A. et al., 1999).

Bellahcene A. et al. (1995) Expression of bone sialoprotein, a bone matrix protein, in human breast cancer. Cancer Res. 54, 2823-2826.
Bellahcene A. et al. (1996a) Bone sialoprotein expression in primary human breast cancer is associated with bone metastasis development. J. Bone Miner. Res. 11, 665-670.
Bellahcene A. et al. (1996b) Expression of bone sialoprotein in primary human breast cancer is associated with poor survival. Int. J. Cancer 69, 350-353.
Fisher L.W. et al. (1990) Human bone sialoprotein: deduced protein sequence and chromosomal localization. J. Biol. Chem. 265, 2347-2351.
Gillespie M.T. et al. (1997) Calcitonin receptors, bone sialoprotein and osteopontin are expressed in primary breast cancers. Int. J. Cancer 73, 812-815.
Hunter G.K. and Goldberg H.A. (1993) Nucleation of hydroxyapatite by bone sialoprotein. Proc. Natl. Acad. Sci. USA 90, 8562-8565.
Kerr J.M. et al. (1993) The human bone sialoprotein gene (IBSP): genomic localization and characterization. Genomics 17, 408-415.
Kim R.H. et al. (1994) Characterization of the human bone sialoprotein (BSP) gene and its promoter sequence. Matrix Biol. 14, 31-40.
Kim R.H. and Sodek J. (1999) Transcription of the bone sialoprotein gene is stimulated by v-Src acting through an inverted CCAAT box. Cancer Res. 59, 565-571.
Raynal C. et al. (1996) Bone sialoprotein stimulates in vitro bone resorption. Endocrinology 137, 2347-2354.
Sharp J.A. et al. (1999) Tumor cells are the source of osteopontin and bone sialoprotein expression in human breast cancer. Lab. Invest. 79, 869-877.
Sung V. et al. (1998) Bone sialoprotein supports breast cancer cell adhesion, proliferation, and migration through differential usage of the alpha_Vbeta₃ and alpha_Vbeta₅ integrins. J. Cell. Physiol. 176, 482-494.
Van der Pluijm G. et al. (1996) Bone sialoprotein peptides are potent inhibitors of breast cancer adhesion to bone. Cancer Res. 56, 1948-1955.